A variety of bioabsorbable polymeric compounds have been developed for use in medical applications. Materials made from these compounds can be used to construct implantable devices that do not remain permanently in the body of an implant recipient. Bioabsorbable materials are removed from the body of an implant recipient by inherent physiological process of the implant recipient. These processes can include simple dissolution of all or part of the bioabsorbable compound, hydrolysis of labile chemical bonds in the bioabsorbable compound, enzymatic action, and/or surface erosion of the material. The breakdown products of these processes are usually eliminated from the implant recipient through action of the lungs, liver, and/or kidneys. It is recognized that in the literature “bioresorbable,” “resorbable,” “bioabsorbable,” and “biodegradable” are terms frequently used interchangeably. “Bioabsorbable” is the preferred term herein.
Absent adhesive binders, adjuncts, or post extrusion melt processing, only non-woven materials with self-cohered filaments that can be captured in a quenched amorphous condition have the requisite intra-fibrillar structure to be considered viable candidates for a stretching process that increases the porosity of the final non-woven web material. A suitable precursor nonwoven web material for stretching below the melting point (Tm) of the foundational bioabsorbable polymer is taught by Hayes in U.S. Pat. No. 6,165,217.
U.S. Pat. No. 6,165,217, issued to Hayes, discloses a bioabsorbable material in the form of a non-woven self-cohered web (FIGS. 1 and 1A, herein). A self-cohered non-woven web material is a spun web of continuous filaments made of at least one semi-crystalline polymeric component covalently bonded as a linear block copolymer with or blended with one or more semi-crystalline or amorphous polymeric components.
The continuous filaments are produced by selecting spinning conditions that provide a tackiness to the emerging filaments and allows them to self-cohere as solid filaments as the filaments are collected in a cohesive random pile, or web, on a collecting surface. The spun filaments are intermingled together as they are collected in the form of a porous web of self-cohered filaments. The self-cohered filaments have multiple contact points with each other within the web. The self-cohered filaments bond at the contact points without need for requisite addition of supplementary adhesives, binders, adhesive adjuncts (e.g., solvents, tackifier resins, softening agents), or post extrusion melt processing. The self-cohered filaments of the preferred embodiment polyglycolide:trimethylene carbonate (PGA:TMC) non-woven web are between 20 microns and 50 microns in diameter. According to Hayes, these self-cohered non-woven webs possess volume densities (also reported as apparent densities) that indicate percent porosity to be in a range between approximately forty (40) and eighty (80). If the potentially semi-crystalline web is preserved in a thermodynamically unstable (metastable), homogeneous (microphase disordered), substantially phase miscible, amorphous state of limited crystallinity, the web is malleable and can be readily conformed or molded into a desired shape. That shaped form can then be preserved through its conversion into a more ordered, thermodynamically stable, at least partially phase immiscible semi-crystalline state. This irreversible (short of complete remelting and reformation of the formed web structures) conversion from a prolonged amorphous (i.e., disordered state of miscibility) condition into an ordered semi-crystalline state is typically provided by the chain mobility present in the rubbery state existing between the melt temperature and that of the order-disorder transition temperature (Todt), the temperature above which the transition from disorder to order can proceed. Alternatively, solvents, lubricants, or plasticizing agents, with or without their combination with heat, can be used to facilitate chain mobility, and rearrangement of the constituent polymer chains into a more ordered condition. The chemical composition of the self-cohered filaments can be chosen so the resultant web is implantable and bioabsorbable.
Hayes describes the self-cohered non-woven web material as possessing a degree of porosity variable based on fiber deposition density and any subsequent compression. Hayes also describes the ability of the planar web in the malleable unstable amorphous condition to be shaped into a virtually unlimited array of forms, the shapes of which can be retained through subsequent crystallization. However, Hayes does not indicate an unset web of the self-cohered filaments which can serve as a precursor web material for additional stretch processing to increase web porosity prior to annealing. Nor does Hayes teach a self-cohered non-woven web material having a significant population of continuous filaments with a cross-sectional diameter less than twenty (20) microns. In the absence of additional processing of a precursor web material according to the present invention, the self-cohered non-woven web material of Hayes would not have increased molecular orientation in the self-cohered filaments of the web sufficient to provide a birefringence value greater than 0.050.
A non-woven self-cohered web material having high porosity and small filament diameter would have proportionally increased mechanical strength in one or more directions. Despite increased mechanical strength, such a high porosity non-woven self-cohered web material would deliver more loft, suppleness, drapability, conformability, tissue compliance, and hæmostatis than a web material made according to Hayes.
For non-implantable applications, a non-woven self-cohered web having a high degree of porosity could be used to releasably attach implantable devices and materials to a delivery apparatus. Combining a population of oriented filaments with an increased internal void volume within which the oriented filament can move would imbue such a material with a degree of elasticity or resiliency.
In addition to these and other improvements in such a web material, a more porous bioabsorbable web material would provide opportunities to combine other components with the web. The components could be placed on surfaces of the filaments. The components could also be placed within void spaces, or pores, between the filaments. The components could be bioabsorbable or non-bioabsorbable. The components, in turn, could releasably contain useful substances.
There is a need, therefore, for a synthetic bioabsorbable, non-woven, self-cohered polymeric web material having a high degree of porosity with increased mechanical strength, loft, suppleness, drapability, comformability, and tissue compliance.